As a waste water treatment method, there has been used a membrane separation activated sludge method in which a membrane cartridge is immersed in an activated sludge tank and a solid-liquid separation is carried out between the activated sludge and the treated water after treatment. This method allows the concentration of the activated sludge (MLSS: Mixed Liquor Suspended Solid) to be set at a very large value of from 5,000 to 20,000 mg/l in the filtering process. This advantageously allows the capacity of the activated sludge vessel to be reduced or enables a reaction time in the activated sludge vessel to be shortened. Further, filtration with the membranes prevents suspended solids (SS) from being mixed into the treated water, thus eliminating the need for a final sedimentation tank. This makes it possible to reduce the construction area of the treatment facility and to achieve filtration regardless of whether or not activated sludge is appropriately sedimented. This method thus has advantages such as a reduction in the burden of activated sludge control. Therefore, in recent years, the membrane separation activated sludge method has rapidly prevailed.
If hollow fiber membranes are used for the membrane cartridge, the high strength of the membranes themselves reduces the chances of the surfaces of the membranes from being damaged as a result of contact with contaminants contained in the raw water. The membrane cartridge can thus be used for a long period. Moreover, this structure has the advantage of being capable of being back washed by reverse filtration. That is, by injecting a medium such as treated water in a direction opposite to that of filtration, fouling can be removed from the surfaces of the membranes. In this case, however, the effective area of the membranes may be reduced unless the filtration is carried out while preventing aggregates of activated sludge as well as contaminants in the raw water from accumulating in the gap between the hollow fiber membranes. As a result, filtration efficiency will be lowered, thus preventing a stable filtration from being maintained over a long period.
The bottom of the membrane cartridge has been conventionally aerated in order to avoid sludge or the like from accumulating on the surfaces of the hollow fiber membranes or between the hollow fiber membranes. This causes the membranes to oscillate and agitating bubbles to rise upward, thus removing any aggregates of activated sludge as well as contaminants in the raw water from the surfaces of the hollow fiber membranes or from between the hollow fiber membranes. Consequently, aggregates and contaminants are prevented from accumulating. For example, a lower ring (or skirt) can be installed at the bottom of the hollow fiber membrane cartridge. Further, a plurality of through-holes can be formed in a lower ring-side adhesion fixation layer. Then, the bottom of the cartridge is aerated to form a pool of air in the end of the lower ring projecting from the lower ring. Thus, bubbles are uniformly generated in the plurality of through-holes. This allows easy removal of suspended substances that have accumulated on the outer surfaces of the hollow fiber membranes (see, for example, JP-A-2000-157846).
If a high concentration MLSS is filtered by the membrane separation activated sludge method, sludge can be released from between the hollow fiber membranes by agitation based on aeration as well as oscillation of the membranes. However, in this case, the rising bubbles exert a force on the aggregates of activated sludge and the contaminants that drive them upward. In a membrane bundle near the cartridge head, not all of the aggregates and contaminants slip from the membrane bundle. These aggregates and contaminants accumulate gradually. A further increase in the amount of aeration increases the number of sludge aggregates and contaminants passing between the hollow fiber membranes. As a result, more aggregates and contaminants accumulate in the bundle near the cartridge head. This may cause the surfaces of the hollow fiber membranes to be blocked.